The peptide substance P decreased the turnover rate of acetylcholine in hippocampus and septum without changing the levels of either acethylcholine or choline. Somatostatin, neurotensin and TRH, all pituitary neurohormones, interfered with the cholinergic nervous system. All three peptides enhanced the TRACh in some brain areas, but the profile of activity differed for different peptides, indicating a specific role in neuronal regulation. Angiotensin II given intraventricularly to rats, increased the blood pressure and induced drinking behavior in accordance with previous observations. In addition angiogensin II decreased specifically the level of acetylcholine in cortex, but did not change the turnover rate under the conditions used. The present results suggest the presence of specific receptor sites for peptides in different parts of the brain, which play an important role in regulation of cholinergic neurons. BIBLIOGRAPHIC REFERENCE: Cheney, D.L., Moroni, F., Malthe-Sorenssen, D. and Costa, E.: Endogenous modulators of acetylcholine turnover rate. In Jenden, D.J. (Ed.): Cholinergic Mechanisms and Psychopharmacology. New York, Plenum Press, 1977, in press.